Direct current motors have versatility in a wide range of applications. The availability of a battery power source for dc motor equipped devices facilitates a portability aspect that is not readily available for a-c motor drives. Electronic controls, such as microprocessor based systems, for a wide variety of functional applications have become commonplace. As development of the battery has progressed, and the versatility of electronic controls has expanded, the challenge of providing efficient direct current motor drives for vehicles, as a viable alternative to combustion engines, has become more compelling. U.S. Pat. No. 5,164,623 to Shkondin is one example of a proposed implementation in which a motor is mounted on the wheel of a vehicle for directly driving the vehicle. The patent proposes that such an arrangement can be applicable to electric cars, bicycles, wheelchairs and the like.
Electronically controlled energization of windings of direct current motors offers the prospect of more flexible management of motor characteristics. The use of permanent magnets in conjunction with such windings is advantageous in limiting current consumption. U.S. Pat. No. 4,754,207 to Heidelberg et al. describes a direct current motor having a rotor composed of a continuous ring of a plurality of permanent magnets successively alternating in polarity. The stator, comprising a plurality of electronically switchable electromagnet poles, is circumferentially separated from the rotor magnets by a radial air gap. Several adjacent stator electromagnets form a phase group. The inward base portions of adjacent electromagnet poles in each group are in surface area contact with each other to form a continuous magnetic flux path. The electromagnetic circuit is broken at transition points between adjacent groups of electromagnets. Sensors detect relative rotational position between rotor and stator elements to control electronic switching of the individual electromagnet windings. Electromagnets belonging to a common group are switched simultaneously with one common electronic switching means per group. Windings of the electromagnets in adjacent groups are of different phases and are switched at different times.
Of concern in implementation of stator winding switched energization is the avoidance of unfavorable consequences such as rotation irregularities. For example, simultaneous switching of all motor phase windings can cause pulsating output torque. Alleviation of these effects, with varying success, can be obtained by appropriately switching all phases at different times or by simultaneously switching certain winding combinations that are distributed symmetrically about the stator periphery and bear certain positional relationships with the permanent magnet poles of the rotor. However, switching of adjacent windings at different times leads to detrimental effects if the windings are linked to a continuous magnetic circuit path, as the flux changes due to the changed energization of the winding of one pole effects the flux of an adjacent pole.
Heidelberg et al. alleviates this problem to some extent by grouping pluralities of stator poles in separate magnetic circuit paths. The magnetic circuit path discontinuity between adjacent groups effects an isolation of magnetic flux, thus reducing transformer like magnetic flux interference between groups. However, where all poles of a group are wound and switched simultaneously, a torque ripple effect can still exist. Heidelberg et al. provides modifications in which some poles of a group are not wound and/or the pole structure of all poles within a group are not of uniform configuration, thus deterring the effects of torque ripple and flux interference between adjacent poles. Such modifications sacrifice torque characteristics. If fewer poles is are wound, flux generation capability is reduced. The unwound poles do not contribute to torque and can negatively interact with rotor permanent magnets. Non-uniform pole configuration modifications in Heidelberg et al. are coupled with non-uniform pole windings. Such configurations complicate the manufacturing process and compromise motor efficiency.
The need remains for an improved motor amenable to simplified manufacture and capable of efficient flexible operating characteristics. In the particular vehicle drive environment, it is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability at minimum power consumption. Such a vehicle motor drive should advantageously provide ready accessibility to the various structural components for replacement of parts at a minimum of inconvenience.